More precisely, the present invention relates to the problem of heterogeneous deformations that appear during transfer of a layer from a substrate termed the “donor substrate” onto a final substrate termed the “receiving substrate.” Such deformations have in particular been observed in the context of three-dimensional component integration technology (3D integration), which requires the transfer of one or more layers of microcomponents onto a final support substrate, and also in the context of circuit transfer or in the production of back-lit imaging devices. The transferred layer or layers include the microcomponents (electronic, optoelectronic, etc.) produced at least in part on an initial substrate, the layers then being stacked onto a final substrate that may possibly itself include components. Particularly because of the very reduced size and large number of the microcomponents present on the same layer, each transferred layer must be positioned on the final substrate with great accuracy in order to be in very strict alignment with the subjacent layer. Furthermore, it may be necessary to carry out treatments on the layer after transfer thereof, for example, in order to form other microcomponents, to uncover the microcomponents on the surface, to produce interconnections, etc.
However, the Applicant has observed that, after transfer, there are situations in which it is very difficult or even impossible to form additional microcomponents in alignment with the microcomponents formed before the transfer.
This phenomenon of misalignment is described with reference to FIGS. 1A to 1E that illustrate an example of producing a three-dimensional structure comprising transfer, onto a final substrate, of a layer of microcomponents formed on an initial substrate, and the formation of an additional layer of microcomponents on the exposed face of the initial substrate after bonding. FIGS. 1A and 1B illustrate an initial substrate 10 with a first series of microcomponents 11 formed thereon. The microcomponents 11 are formed by photolithography using a mask in order to define zones for the formation of patterns corresponding to the microcomponents 11 to be produced.
As can be seen in FIG. 1C, the face of the initial substrate 10 comprising the microcomponents 11 is then brought into intimate contact with one face of a final substrate 20. Bonding between the initial substrate 10 and the final substrate 20 is generally carried out by molecular bonding. Thus, a buried layer of microcomponents 11 is obtained at the bonding interface between the substrates 10 and 20. After bonding and as can be seen in FIG. 1D, the initial substrate 10 is thinned in order to withdraw a portion of the material present above the layer of microcomponents 11. Thus, a composite structure 30 is obtained, formed by the final substrate 20 and a layer 10a corresponding to the remaining portion of the initial substrate 10.
As can be seen in FIG. 1E, the next step in producing the three-dimensional structure consists in forming a second layer of microcomponents 12 at the exposed surface of the thinned initial substrate 10, or of carrying out additional technical steps on that exposed surface in alignment with the components included in the layer 10a (contact points, interconnections, etc.). For the purposes of simplification, in the remainder of the text, the term “microcomponents” is applied to devices or any other patterns resulting from the process steps carried out on or that are in layers where the positioning must be accurately controlled. They may thus be active or passive components, with a single contact point or with interconnections.
Thus, in order to form the microcomponents 12 in alignment with the buried microcomponents 11, a photolithography mask is used that is similar to that used to form the microcomponents 11. The transferred layers, like the layer 10a, typically include marks, both at the level of the microcomponents and at the level of the wafer forming the layer, that are used by the positioning and alignment tools during the processing steps such as those carried out during photolithography.
However, even when positioning tools are employed, offsets are produced between certain of the microcomponents 11 and 12, such as the offsets Δ11, Δ22, Δ33, Δ44, indicated in FIG. 1E (respectively corresponding to the offsets observed between the pairs of microcomponents 111/121, 112/122, 113/123, 114/124, etc.).
Such offsets do not result from elementary transformations (translation, rotation or combinations thereof) that could originate from an inaccurate assembly of the substrates. These offsets result in non-homogeneous deformations that appear in the layer originating from the initial substrate during its assembly with the final substrate. In fact, such deformations cause local, non-uniform movements at certain microcomponents 11. In addition, certain of the microcomponents 12 formed on the exposed surface of the substrate after transfer have positional variations with the microcomponents 11 that may be of the order of a few hundred nanometers or even a micrometer.
The phenomenon of misalignment (also termed “overlay”) between the two layers of microcomponents 11 and 12 may be a source of short-circuits, distortions in the stack or connection defects between the microcomponents of the two layers. Thus, when the transferred microcomponents are imaging devices formed by pixels and the post-transfer treatment steps are intended to form color filters on each of these pixels, a loss of the colorization function has been observed for some of the pixels.
The phenomenon of misalignment thus results in a reduction in the quality and value of the multilayer semiconductor wafers produced. The impact of that phenomenon is becoming much greater due to the constant demand for miniaturization of microcomponents and their integration density per layer.
Alignment problems during the manufacture of three-dimensional structures are well known. The document by Burns et al., “A Wafer-Scale 3-D Circuit Integration Technology,” IEEE Transactions on Electron Devices, vol. 53, No. 10, October 2006, describes a method of detecting alignment variations between bonded substrates. The document by Haisma et al., “Silicon-Wafer Fabrication and (Potential) Applications of Direct-Bonded Silicon,” Philips Journal of Research, Vol. 49, No. 1/2, 1995, emphasizes the importance of wafer flatness, in particular during the polishing steps, in order to obtain good quality final wafers, i.e., with the smallest possible number of offsets between the microcomponents. However, those documents deal only with the problem of positioning the wafers during assembly thereof. As explained above, the Applicant has observed that even when the alignment between two wafers is perfect while they are being brought into contact (using marks provided for that purpose), non-homogeneous movements of certain microcomponents occur following initiation of the bonding wave.